This invention relates to a process for the deriming of heat exchangers at low temperatures, and, in particular, deriming heat exchangers which are contaminated with mercury, such as those used for liquefaction of natural gas.
The liquefaction of natural gas for storage and transportation and regasification for final distribution is a well established technology. Liquefied natural gas (LNG) represents an economically attractive energy option, especially for industrial nations short on domestic fuel reserves.
Several types of natural gas liquefaction processes are known. One conventional LNG process, the standard cascade system, uses three different refrigerants, i.e., methane, ethylene and propane, circulating in closed cycles. An example of such a system is described in U.S. Pat. No. 3,593,535. An improvement over the standard cascade system employs a single-pressure mixed refrigerant cascade (MRC) system. In one version of the MRC system described in Geist et al., "Predicted and Actual Temperature Profiles And Pressure Drops in Large Coil Wound, Mixed Refrigerant Heat Exchangers," LNG6, Session II, Paper 4, Apr. 7, 1980, Kyoto, Japan, a natural gas feed following treating and drying is precooled in an auxiliary heat exchanger supplied with propane refrigerant. Thereafter, the chilled gas is introduced into a cryogenic main heat exchanger (MHE) where liquefaction takes place. The MHE is horizontally divided into an upper cold bundle absorbed by propane and a lower warm bundle absorbed by mixed refrigerant.
In any natural gas liquefaction process, there will be progressive accumulation of water, as ice, and relatively heavy hydrocarbons present in the gas feed in solid form upon the interior surfaces of the MHE. At some point the accumulation will be sufficient to hinder the efficient operation of the process. In particular, there will be an increased pressure drop and a decrease of the heat transfer rate along the surfaces of the MHE. Consequently, a deriming, or defouling procedure will be required to restore the efficiency of the liquefaction process to some acceptable level. Conventional deriming practice calls for warming the heat exchanger to about 60.degree.-70.degree. C. and purging it with dry nitrogen gas.
Regardless of the liquefaction system used, aluminum is often the material of choice for the construction of the cryogenic heat exchanger due to its high thermal conductivity, excellent low temperature properties, machinability and relatively low cost. However, aluminum is susceptible to corrosion by mercury which is present in natural gas, e.g., from as low as about 0.005 to as high as about 200 micrograms per normal cubic meter (i.e., from about 5.5.times.10.sup.-3 to about 220 parts per billion by volume). Concentrations of mercury greater than about 0.01 micrograms per normal cubic meter are generally regarded as undesirable especially where aluminum cryogenic liquefaction equipment is concerned due to mercury's capability for forming a corrosive amalgam with aluminum. Although it is a conventional practice to demercurate natural gas (see, for example, the demercuration processes described in U.S. Pat. Nos. 3,193,987; 3,803,803; 4,101,631; 4,474,896; 4,491,609; 4,474,896; and, 4,500,327), a sufficient amount of elemental mercury will often remain in the post-treated gas as to pose a significant safety and maintenance problem where aluminum cryogenic heat exchangers are concerned. Thus, during the aforedescribed conventional deriming procedure, warming of the heat exchanger to 60.degree.-70.degree. C. causes melting of any solid elemental mercury which may be present therein thus permitting the mercury to destroy the heat exchanger through embrittlement and amalgamation with the aluminum.
It is an object of the present invention to provide a process for deriming cryogenic equipment employed in the liquefaction of natural gas.
It is a particular object of this invention to carry out the deriming of cryogenic heat exchangers fabricated from aluminum in a manner which minimizes the corrosive potential of elemental mercury vapor, a normal component of natural gas, for the aluminum.